NetEq4: Small change to reduce allocs in AudioMultiVector

This change reduced the allocation count by 20000 in the bit-exactness
test.

BUG=Issue 1363
TEST=out/Debug/modules_unittests --gtest_filter=NetEqDecodingTest.TestBitExactness
R=turaj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2157004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@4678 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed